Cell surfaces can be modified in order to modulate surface function or to confer new functions to such surfaces. Surface functionalization may, for example, include an addition of a detectable label, a binding moiety, or a therapeutic agent to a surface protein, allowing for the detection or isolation of the surface-modified cells, for the generation of new cell-cell interactions that do not naturally occur, or for the conjugation of a therapeutic or diagnostic agent to the cell surface.
Cell surface modification can be achieved by genetic engineering or by chemical modifications of cell surface proteins. Both approaches are, however, limited in their capabilities, for example, in that many surface proteins do not tolerate insertions above a certain size without suffering impairments in their function or expression, and in that many chemical modifications require non-physiological reaction conditions.